Factors that regulate the elongation phase of RNA polymerase II (RNAPII) transcription also play an important role in protecting cells from DNA damage and environmental stress. Inhibition of transcription elongation activates the p53 tumor suppressor to signal a stress response, even in the absence of DNA damage (Derheimer. et al., Prot Natl Acad Sci 104:12778-12783 (2007); Gartel, Biochim Biophys Acta 1786:83-86 (2008)). Flavopiridol (FP) promotes apoptosis through induction of p53 and inhibition of short-lived anti-apoptotic proteins, and is currently in clinical trials as an anti-cancer agent for leukemia and solid tumors (Canduri et al., Med Chem 4:210-218 (2008); Wang et al., Mini Rev Med Chem 9:379-385 (2009)). Thus, RNAPII is a genome wide sensor for DNA damage, through its ability to activate p53 and initiate programmed cell death upon encountering significant blocks to elongation.
The Ski-interacting protein SKIP (Snw1 and NCoA62) is a transcriptional coactivator for many newly induced genes (Leong et al. 2001, 2004; Zhang et al. 2003; Folk et al. 2004; MacDonald et al. 2004) and counteracts transcriptional repression by retinoblastoma (Prathapam et al. 2002). The SKIP homologs in Saccharomyces cerevisiae (Prp45) and Drosophila (BX42) are essential for cell viability, splicing (Ambrozkova et al., Biochem Biophys Res Common 284:1148-1154 (2001); Makarov et al., Science 298: 2205-2208 (2002); Gahura et al., J Cell Biochem 106:139-151 (2009)), and nuclear export of spliced mRNAs (Farny et al., Genes Dev 22:66-78 (2008)). Although elongation factors can affect splicing indirectly through changes in the rate of elongation, and defects in cotranscriptional splicing can reduce RNAPII elongation rates in vivo (Kornblihtt Adv Exp Med Biol 623: 175-189 (2007); Pirngruber et al., Cell Cycle 8:3636-3642 (2009)), SKIP is recruited to promoters as well as transcribed regions and appears to play a direct role in each process.
Different subsets of p53 target genes specify whether cells will arrest to repair DNA damage, or undergo apoptosis (Vazquez et al., Nat Rev Drug Discov 7:979-987 (2008); Vousden et al., Cell 137:413-431 (2009)). Key p53 target genes in these opposing pathways are the anti-apoptotic G1 cell cycle arrest factor p21 (Abbas & Dutta, Nat Rev Cancer 9:400-414 (2009)) and the proapoptotic BH3-only Bcl-2 protein PUMA. The relative levels of these two proteins help to determine the extent of cell survival in response to DNA damage (Yu & Zhang, Cancer Cell 4:248-249 (2003); Iyer et al., Proc Natl Acad Sci 101:7386-7391 (2004)). Known transcription factors that impact this balance include c-Myc, which represses p21 without affecting PUMA expression (Seoane et al., Nature 419: 729-734 (2002); Jung et al., Cell Cycle 8: 982-989 (2009)), and the bromodomain protein Brd7, which promotes p53 binding to the p21, but not PUMA, gene (Drost et al., Nat Cell Bio 112:380-391 (2010)). Cell growth arrest arising from rapid p21 induction is an initial protective response to DNA damage or oncogene expression. Although the p21 gene is predominantly regulated at the level of transcription, additional factors control its translation as well as protein and mRNA stability.
Cancer is widely recognized as one of the major challenges to the healthcare industry, in terms of the variety of specific disease processes embraced by the term, the number of people and animals afflicted, and the effort and resources devoted to its treatment. For years, cancer has resisted attempts to understand and control the disease. The major, broad-based therapeutic approaches to cancer treatment continue to be burdened by deleterious side effects. For example, chemotherapy involves the delivery of cytotoxic compounds that target dividing cells to thereby destroy cancer cells. Healthy dividing cells are also lost, however, and the treatments can lead to serious, life-threatening complications. The treatments frequently result in pain, nausea, hair loss, and a highly increased risk of serious infection. Radiotherapy, another broad-based approach, also imperfectly targets cancer cells, with the result that healthy as well as cancerous cells can receive a lethal dose of radiation, leading to side effects such as pain, loss of vigor, and an increased risk of secondary malignancies, up to 20%, in some cases.
The inventors have discovered a novel target for preferentially inducing cell death in cancer (or pre-cancer) cells, or preferentially reducing DNA damage-induced cell death in non-cancer cells. Thus, provided herein is a broad-based therapeutic strategy for addressing cancer and the side effects of existing cancer treatments.